Two different techniques can be used to change the direction of rotation, Figure 6.4.
Notice that twisting the belt changes the direction of rotation. This technique is acceptable if the pulleys are some distance apart, the belt is operating at slow speed and it is not transmitting a large load. The belts will rub at the center and these three conditions will increase the amount of friction at that point. The friction produced from rubbing will shorten the life of the belt.
The addition of a third pulley also will reverse the direction of rotation. In this situation, a six-sided, or double-V, belt will be used if the third pulley requires a lot of torque, if not, a flat pulley will be used that runs on the back side of the belt. This is very common in serpentine drives used on modern automobiles.
The use of the third pulley also provides an opportunity to change the speed of rotation. A difference in diameter between the third pulley and the driver will cause a change in speed.
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